ALKENE POLYMER

RUBBERY CYCLO-OLEFIN (CYCLO-ALKENE) POLYMERS... 

Rubbery Cyclo-olefin (Cyclo-alkene) Polymers 305... 

We had a brief introduction to radical reactions in Section 5.3 and said at that time that radicals can add to alkene double bonds, taking one electron from the double bond and leaving one behind to yield a new radical. Let s now look at the process in more detail, focusing on the industrial synthesis of alkene polymers. 

Table 7.1 shows some commercially important alkene polymers, their uses, and the vinyl monomers from which they are made. 

Table 7.1 Some Alkene Polymers and Their Uses...

Alkene polymers—large molecules resulting from repetitive bonding together of many hundreds or thousands of small monomer units—are formed by reaction of simple alkenes with a radical initiator at high temperature and... 

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4-addition of the growing chain to a conjugated diene monomer. 

Chain-growth polymer (Section 31.1) A polymer w hose bonds are produced by chain reactions. Polyethylene and other alkene polymers are examples. 

Ziegler-Natta catalyst (Section 31.2) A catalyst of an alkylaluminum and a litanium compound used for preparing alkene polymers. 

The first alkene polymer to be used in society was polyisoprene, a natural product extracted from the sap of rubber trees. See our Box for a description of the history of rubber. The monomer from which this polymer is constructed... 

Alkene polymers such as poly(methyl methacrylate) and polyacrylonitrile are easily formed via anionic polymerization because the intermediate anions are resonance stabilized by the additional functional group, the ester or the nitrile. The process is initiated by a suitable anionic species, a nucleophile that can add to the monomer through conjugate addition in Michael fashion. The intermediate resonance-stabilized addition anion can then act as a nucleophile in further conjugate addition processes, eventually giving a polymer. The process will terminate by proton abstraction, probably from solvent. 

Other addition polymerizations were discussed previously, including poly-l,3-cyclopentadiene, alkene polymers (Section 10-8), polyalkadienes (Section 13-4), polyfluoroalkenes (Section 14-7D), and polymethanal (Section 16-4B). 

These striking examples of suburban yard art are made of polypropylene, an alkene polymer. 

Polymers are large molecules formed when many smaller monomers bond together. Alkene polymers such as polyethylene result from the polymerization of simple alkenes. Nylons and polyesters result from the sequential reaction of two difunctional molecules. 

The following structure represents a segment of an alkene polymer. Identify the monomer from which the polymer was made. 

Show the monomer units you would use to prepare the following alkene polymers ... 

Dehydrogenation / reforming Pt/Al203 alkanes alkenes polymers, bulk chemicals... 

Polyfvinyl alcohol) is an important example. Inspection of the structure reveals that this is a typical alkene polymer but the monomer would have to be vinyl alcohol—the unstable enol of acetaldehyde. The way to make the polymer is to start with something else and only later... 

Grubbs [3] prepared high activity metathesis ruthenium metal carbene complexes, (IV), that were effective as depolymerization catalysts of unsaturated polymers and synthetic agents in preparing telechelic and alkene polymers. Other high activity metathesis ruthenium carbene metal complexes, (V), were prepared by Fogg [4]. 

Rubber—an unusual name for a nuM unusual substance—is a naturally occurrifxf alkene polymer produced by more than 400 diflerent plants. The iilAjer saum is tlie Mj-eal>ed rubber tree. Hetva brasiiiensi, from whieh the crude material is harvested as H drips from a dice made thraugh the bark. The name rubber was coined by Joseph Priestley, the discoverer of oxygen and early researcher of rubber chemistry, for the simple reason that one of rubber s early uses was to rub out pencil marks on paper. 

The common metathesis reactions for the preparation of metallocenes, treating a metal salt MX2 with NaCp, are hampered in the case of ruthenium by the lack of suitable Ru salts. (Rul2 is commercially available, but is still not commonly used in the synthesis of rathenocene.) Thus, ruthenocene has been obtained from Ru(acac)3 and NaCp in very low yield and later from RuCb and NaCp in 50-60% yield. It has now become apparent that alkene polymers, in particular [Ru(nbd)Cl2]x, but also [Ru(cod)Cl2]x and hydrazine derivatives (Section 3.1), can serve as Ru precursors. Equally successful in many cases is reductive complexation of cyclopentadiene in ethanol in the presence of Zn (Section 3.2), which furnishes the metallocene in about 80% yield. Decamethylruthenocene (82) was first obtained by the Zn reduction route in 20% yield, but can now be prepared conveniently from halide complexes [Cp RuCl2]2 or [Cp RuCl]4, a common method for the preparation of symmetrical and unsymmetrical sandwich compounds of ruthenium featuring one alkyl-substituted ligand. 

Unlike polyethylene and other simple alkene polymers, natural rubber is a polymer of a diene, iso )re ie (2-methyl-l,3-butadiene). The polymerization takes place by addition of isoprene monomei units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon inti r-vals. As the following structure shows, these double bonds have Z stereochemistrv ... 

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